[Mag] Keyboard shortcut info shown when physical keyboard present

Shows keyboard shortcut info in magnification settings when a
physical keyboard is present. See bug for screenshots.

Hides touchscreen info in magnification settings when a touchscreen
is not present.

Adds ShadowInputDevice support for physical full keyboards.

Bug: b/388847050
Test: Manual, atest ToggleScreenMagnificationPreferenceFragmentTest
Flag: com.android.server.accessibility.enable_magnification_keyboard_control
Change-Id: Ib53fbd8f929d1cc8e294f6f04bab405c9bb576a9